1. Field of Invention
This invention relates to a control device for a hybrid vehicle, which is controlled so that the total output of an engine and a motor becomes the requested output of the driver.
2. Description of Related Art
In recent years, due to consideration of environmental problems, various types of hybrid vehicles, which include an engine and a motor as driving sources, have been provided. Among these hybrid vehicles, a so called directly connected parallel type hybrid vehicle, which, for example, connects a rotor of the motor to a crankshaft of the engine and inputs a driving force of the engine and the motor into an automatic transmission, has been proposed (Japanese laid open patent H9-215270).
An automatic transmission sometimes requires limitation of the engine torque, so that the engine torque becomes the torque available for input to the automatic transmission, to protect the automatic transmission. Such occasions include, for example, a situation when the capacity of a clutch to transmit a driving force is reduced due to a torque amplification effect of a torque converter at the time of the start of the vehicle and due to a reduced oil pressure at the time of low engine revolution and a situation when a capacity of a lock-up clutch to transmit a driving force is reduced at the time of reducing a vehicle speed.
In such an engine torque limitation, particularly when the engine torque is larger than a limitation torque, for example, the engine torque can be reduced to the limitation torque by delaying ignition timing of the engine. However, the reduction of the engine torque by delaying ignition timing destabilizes a combustion condition of the engine and generates unburned gas. Further, it is feared that it has adverse effects on auto emissions.
Furthermore, in the above mentioned hybrid vehicle, generally, the vehicle runs by the driving force of the engine and is assisted by the motor when the requested output of the driver is large. The motor operates in a regeneration mode when the vehicle decelerates. In other words, the motor is used secondarily to the engine. In such a hybrid vehicle, according to the output ability of the motor, the engine is driven so that the engine operates in the best (i.e., the most appropriate) condition for fuel consumption, on the basis of the revolution speed of the engine, regardless of a requested output of the driver. The motor outputs the amount necessary to compensate for an insufficient engine output in comparison to the requested output. On the other hand, when the engine output is large, the motor operates in the regeneration mode when the amount of the engine output is greater than the requested output. In this way, the vehicle can provide the requested output of the driver.
However, when the abovementioned limitation of the engine torque is performed, the most appropriate condition for fuel consumption based on the revolution speed of the engine is not achieved. Therefore, an improvement in fuel consumption is not provided.